Another factor comes to mind: UIs are places you spend a fair amount of time, and repetitively. Whilst there are “new” UI elements in an OS from time to time, you in fact see the same ones over and over, much more often. Users of OSs also tend to have time on their side when interacting with the UI, when compared with drivers reading road signs. My take is that you don’t need to optimise for legibility as strongly when designing for an OS UI as you do when designing for road signs.